A variety of compositions and methods have heretofore been used or proposed for use for applying coatings on metal surfaces possessed of lubricating properties to facilitate subsequent metal forming or working operations such as drawing, forging, and the like. Chemical conversion coatings, especially phosphate coatings, have received widespread acceptance for this purpose. Such phosphate conversion coatings are employed in conjunction with supplemental organic lubricating agents such as waxes and soaps to further enhance the lubricity characteristics of the coating formed. It has heretofore been conventional to first subject the metal surfaces to a phosphating treatment to form the requisite phosphate coating thereon, whereafter the parts are water rinsed and thereafter are passed into a lubricant tank for applying the supplemental organic lubricant thereto. The problems associated with such a multiple step pretreatment prior to metal forming operations have somewhat been overcome by coating formulations providing a one-step procedure in forming the necessary lubricant coating thereby substantially reducing the treating time, plant space required, investment in capital equipment and labor required.
Typical one-step methods for forming a lubricant coating on metal surfaces are those disclosed in U.S. Pat. Nos. 2,840,498; 2,850,418; and 3,525,651. In accordance with the methods and compositions disclosed in the aforementioned patents, a lubricating agent is emulsified or dispersed in an aqueous solution containing the conversion coating constituent whereby a conversion coating and a deposition of the lubricant additive is simultaneously effected in a so-called one-step operation. At the completion of the coating operation, the surfaces of the metal parts are dried prior to initiation of the metal forming operations.
A continuing problem associated with one-step processes of the types heretofore known has been the formation of rust on the metal surfaces during the drying stage of the process. The presence of water vapor and the high free acid of the emulsion on the metal parts tends to cause rusting particularly in areas and locations which are difficult to reach by the heated recirculating air employed during the drying operation. Such a situation frequently arises when bulk quantities of metal parts are processed in bundles and the points of contact between adjacent work pieces precludes adequate entry of drying air. The presence of such rust not only results in an unacceptable surface on the finished product but also substantially increases resistance during the metal working or drawing process increasing die wear and resulting in improperly worked or fractured metal parts.
It has now been discovered that rusting of the metal surfaces during the drying stage can be substantially inhibited by the inclusion of a controlled effective amount of a specific type of rust inhibiting agent in the operating bath without significantly affecting the stability of the emulsion. It has been observed, however, that during use of the bath, iron is dissolved from the metal surfaces processed causing a progressive increase in iron concentration in the bath which tends to reduce the effectiveness of the rust inhibiting agent and promotes emulsion instability.
The present invention overcomes the problems and disadvantages associated with prior art one-step lubricant coating compositions and methods by providing a concentrate useful when diluted with water for forming an operating bath which is of versatile use, effective to produce a satisfactory lubricant conversion coating on a metal surface, which is substantially stable over a broad range of operating conditions, which has a prolonged operating life and which inhibits the formation of rust during the drying stage.